When Marco first met his new foster father, his reaction was one of relief. Not yet six, he had taken to wandering the streets of West Berlin to escape an abusive father who beat him so hard that his legs would bleed.

By contrast, Fritz Henkel — a man in his late forties whom he soon came to call “Papa” or “Paps” — was kindly, and his five-bedroom flat in the city’s Friedenau district was enormous. Marco had his own bedroom, a basic computer on which to play games, and two pet rabbits.

Unwittingly, however, he had become the subject of a bizarre experiment carried out from the late 1960s until well into the 2000s. Marco and as many as 200 other boys were allegedly put into the care of known paedophiles.

Henkel’s mask quickly dropped: he began to shout at Marco (not his real name) and hit him. After about six months, in the autumn of 1989, shortly before the fall of the Berlin Wall, he came into Marco’s room one evening asking for a cuddle. He then ordered the boy to perform oral sex on him and later raped him.

According to Marco’s account, Henkel continued to abuse him, raping him once a week or so for the next decade, often filming him. The assaults became rarer when Marco was in his teens, finally stopping when he was 17 or 18 and strong enough to resist.

“He was a psychopath, a monster. He had no feeling and no emotions,” Marco said this week in a telephone interview from his home in Brandenburg, outside Berlin. “He saw people as objects.”

More than three decades later, Marco — now 39, with a female partner and four-year-old daughter and six-month-old son of his own — is fighting for compensation from the city’s authorities.

A second victim, known as Sven, 36, who was picked up while living on the streets and sent by social services to live with Henkel in 1991, is pursuing a linked legal action over similar sexual abuse he claims to have suffered.

Henkel, who is now dead, lived off the 12,000 deutschmarks a month (about £4,000) he was given to look after his charges. The former electrician fostered nine children from 1973 until 2003, typically two or three at a time.

They came from troubled families or children’s homes. Some lived rough near West Berlin’s Zoo station — like the street kids whose plight was depicted in the 1981 film Christiane F. In some cases, it has since emerged, Henkel simply took home those boys who took his fancy, only sorting out the paperwork with the state youth welfare office months later.

To hide what was going on, Henkel discouraged his charges from talking or playing with one another, and encouraged them to misbehave at school to convince teachers they were troublemakers. He never took them to the doctor.

“He wanted to make us dependent on him, to make us feel he had saved us from living in a children’s home or amid the dirt on the street,” Marco said. “He indoctrinated us into seeing everyone else as an enemy.”

Both Marco and Sven claim to have been left traumatised by their experiences, unable to learn an occupation or hold down a job.

Another severely disabled boy, known as Sascha, who also lived for many years in Henkel’s flat, was even more unfortunate. Unable to talk and barely capable of walking, he was neglected, and died in 2003 of a lung infection. “Henkel said he would have died anyway,” Marco said.

Henkel tried to get another foster child in Sascha’s place but was refused. No longer receiving any money from the state, he threw out both Marco and Sven, by then in their early twenties, and moved to the countryside.

Shortly before Henkel died of cancer in 2015, Marco visited him in the hospice. “I wanted to see him while he was dying,” he said. “It sounds mean, but I cried with joy. I was happy, in a way.”

Henkel was an odd choice as foster father. He was single and had spent time in jail for minor offences while serving in the army, and during the 1970s was investigated over the sexual abuse of one of his earlier charges. In January 1980, he was officially informed that the investigation had been shelved.

But suspicion of paedophile tendencies do not appear to have counted against Henkel or the other men to whom children were fostered out by the city’s authorities.

On the contrary, it appears to have worked in their favour, in large part thanks to the efforts of a highly influential academic, Helmut Kentler, who began his career in West Berlin before becoming professor of social education at Hanover University and one of the country’s leading sexologists.

Kentler launched a curious experiment in West Berlin in 1969, with the apparent consent of local authorities, in which troubled young street children were put in the care of three paedophiles — Henkel among them.

In an official report for city authorities in 1988, seen by the Berliner Zeitung newspaper, Kentler wrote it was clear the main reason the men “did so much for ‘their’ boy was because they had a sexual relationship with him”.

Kentler, who died in 2008 and is suspected of having himself been a paedophile, spoke not of abuse but of “the advantages for children of having sexual contacts with adults”.

Many more paedophiles could have been involved. Roman Simon, a centre-right politician in the Berlin state parliament, has estimated that as many as 200 children were handed over to such men. “That is the kind of number we are talking about,” he said.

Henkel enjoyed the personal protection of Kentler, who in professional reports in the city archives was rated “very highly” and praised for his “natural talent for pedagogy”. The two men were friends and spent hours on the phone. Marco remembers visiting Kentler’s home in Hanover and has a photograph of himself as a small boy sitting next to him. Kentler did not assault him, though. “I was Henkel’s property,” he said.

Abhorrent as such ideas sound today, Kentler was not acting in a vacuum. Amid the drive for sexual liberation that swept Europe in the late 1960s and 1970s, some campaigners in Germany went further to challenge the traditional taboos against sex with children.

Such ideas were also advocated elsewhere, including in Britain, where the now notorious Paedophile Information Exchange, set up in 1974, campaigned for the abolition of the age of consent, until the group was disbanded a decade later.

Those who run Berlin at present have long since distanced themselves from such bizarre ideas. In 2016, Teresa Nentwig, a political scientist, was asked to go through the files and conduct an inquiry.

Kentler “was convinced that children and young people from families that neglected them could be helped by living with paedophiles”, Nentwig told German television. The men looked after the boys, but in return enjoyed sexual favours. “It was a kind of barter trade.”

Yet the authorities appear reluctant to act on her findings, denying their employees were at fault and insisting they could not have been aware of what was going on in Henkel’s home — an excuse dismissed by Marco and Sven. The two men are trying to pursue the matter through the courts. Refused legal aid, they have launched a crowdfunding campaign to try to cover their lawyers’ bills.

For Marco, the battle is about justice rather than money — and about coming to terms with his past so he can provide a normal life for his own children. “It is not about financial compensation, it is about identifying those in the administration who were responsible, and getting them to apologise,” he said. “Henkel should never have been given children.”